1. Field of the Invention
The present invention relates to a disk drive having a head that reads data written on a recording surface of a disk. In particular, the present invention relates to an apparatus and method for controlling a head unload operation in a disk drive, the apparatus and method being suitable for detecting an error in unloading of the head in a particular retract area located away from the recording surface of the disk.
2. Description of the Related Art
Recent disk drives, for example, hard disk drives, commonly comprise a ramp. The ramp provides a retract area to which the head is retracted (parked). The retract area partly serves as a stopper that locks the head in the retract area, the head being retracted to the retract area. The position of the stopper constitutes a terminal position of the retract area. The ramp is located away from a recording surface of the disk. The head is supported by a suspension. The suspension is formed at a tip of an actuator. The actuator moves in a radially across the disk. The actuator includes a voice coil motor (VCM) that drives the actuator. A tab is formed at a tip of the suspension. Actually, the tab is retracted to the retract area of the ramp, where it is locked by the stopper. However, for simplification, a common description is that the head is retracted to the ramp or the head is locked by the stopper.
The operation of retracting the head to the retract area of the ramp is called an “unload” (head unload). In contrast, the operation of moving the head to the recording surface of the disk is called a “load” (head load). A large number of concentric tracks are formed on the recording surface of the disk. Further, servo information is prewritten (embedded) in each track discretely at regular intervals. The servo information contains position information (a cylinder code) indicating the position of a cylinder (track) on the disk in which the servo information is written.
During a load/unload operation of the head, the head may not be present on the recording surface of the disk. In this state, the head cannot read the servo information written on the disk. In this case, the hard disk drive cannot detect the position of the head on the basis of the position information contained in the servo information. Consequently, the hard disk drive cannot detect the movement speed (head speed) of the head either. However, a back electromotive force voltage (back-EMF-voltage) generated in the voice coil motor when the head is moved represents the speed of the head (the speed of the actuator). Jpn. Pat. Appln. KOKAI Publication No. 2001-307451 discloses a technique to perform a load/unload operation by speed control (this technique will hereinafter referred to as the “conventional technique”). With this conventional technique, for example, in a head unload operation, the head speed is calculated from the back-EMF-voltage of the voice coil motor. Then, feedback control is performed so as to make the head speed detected appropriate. The distance that the head has moved is also detected (calculated) on the basis of the head speed. In a head unload operation, the head must reliably reach the stopper position. Thus, whether or not the head has reached the stopper position (the terminal position of the retract area) is determined on the basis of the head movement distance detected.
However, if the speed is not correctly detected and the speed detected is thus different from the actual one, then the head movement distance is also different from the actual one. In this case, it is impossible to correctly determine whether or not the head has reached the terminal position of the retract area of the ramp. Then, it may be erroneously determined that the head has reached the terminal position of the retract area even through the head has not actually reached the terminal position. If the head unload operation is completed in accordance with this erroneous determination and if for example, the hard disk drive is subsequently externally impacted or vibrated, then the head may fall onto the recording surface of the disk. In general, the head unload operation is performed to shift the head disk drive (disk drive) to an inoperative state. Then, if a host does not provide any commands for a specified time after the head unload operation has been completed, rotation of the disk is often stopped. If the head is present on the recording surface of the disk while the rotation of the disk remains stopped, the head sticks to the disk.